In recent years, research and development of new clean energies have been encouraged along with buildup of environmental awareness, and solar power generation systems using solar batteries for directly converting the optical energy of the sun into an electrical energy are proliferating. Especially, system interconnection solar power generation systems (utility connected PV systems) in which DC power generated by a solar battery is converted into AC power by an inverter and output to a commercial power system are making a great market. The commercial power system (electric utility) will be simply referred to as a “system”, and the inverter used for interconnection (utility connection) as a “system interconnection inverter” hereinafter.
In a solar battery, insulation may be damaged due to some reason to cause a ground fault. Generally, insulation can be checked by measuring the insulation resistance value using an insulation resistance tester. In many cases, however, since the insulation resistance value is measured by routine inspection once for several months or years, a ground fault may be left undetected for a long period. To prevent this, a system interconnection inverter has a ground fault detector for detecting a ground fault of a solar battery.
Many recent system interconnection inverters (utility connected inverters) are so-called transformerless type inverters that use no insulation transformer from the viewpoint of size, mass, performance, and cost. A ground fault detector used in a transformerless type system interconnection inverter (a transformerless type utility connected inverter) generally uses a scheme of detecting a ground current that flows from the solar battery to the commercial power system through the system interconnection inverter because a potential to ground is supplied from the system to the solar battery. An example of this scheme is described in, e.g., Japanese Patent Publication No. 63-49455.
However, this ground fault detection scheme has the following problems. In this scheme, to detect a ground fault generated in a solar battery, a ground current that flows through a circuit formed by solar battery—power conditioner—system—ground—solar battery is detected. This circuit forms when a ground fault occurs in the solar battery. The ground fault is generated by the potential difference (potential to ground) between ground and the ground fault portion of the solar battery. A solar battery sometimes has a portion where the potential to ground is zero or close to zero. When a ground fault takes place at that portion where the potential to ground is zero or close to zero, the ground current is zero or very small, so the ground fault cannot be detected.
As described above, a solar battery has a portion (to be referred to as a “dead region” hereinafter) where a ground fault that has occurred in the solar battery during operation cannot be detected. In activating a solar power generation system, the output voltage of the solar battery shifts from the open voltage to the optimum operating point voltage. Since the potential to ground changes at this time, a ground fault can be detected in some cases. However, if the time for which the detectable ground current flows is short, the ground fault detector of the above scheme cannot detect the ground fault.
In many cases, the system interconnection inverter performs MPPT (Maximum Power Point Tracking) control to extract the maximum output from the solar battery, thereby changing the output voltage of the solar battery. However, the optimum operating point voltage of the solar battery does not largely change. The degree of a change in operating voltage of the solar battery is actually not so large, and no ground fault can be detected yet. Although a ground fault can be detected at the time of activation of the solar power generation system, a ground fault generated after the start of operation cannot be detected, and the solar power generation system may be continuously operated.
When a ground fault is generated in the dead region where the potential to ground is close to zero, there is nothing to worry about electrical shock, though the ground fault is preferably detected at early stage for quick repair.
Additionally, in the system interconnection inverter, although a ground fault can be detected, its position cannot be determined. Furthermore, because of a change in state after ground fault detection, the ground fault position of the solar battery may be unknown even by check operation done later. This also applies when a, DC power supply such as a battery other than a solar battery or a fuel cell is used.